Conventionally, as an engine lubrication system, there exists an engine lubrication system in which an oil supply passageway is formed in a main shaft and a drive shaft of a transmission, respectively, lubricating oil is supplied into the oil supply passageways by a lubricating oil pump and the lubricating oil so supplied is then supplied to sliding surfaces of the shafts over which change-speed gears slide from branch holes formed in the main shaft and the drive shaft, whereby the sliding surfaces are lubricated.
Incidentally, in lubricating the transmission, while it is important to lubricate meshing surfaces of the change-speed gears, conventionally, it is a generally adopted method in which part of the change-speed gears is submerged in lubricating oil stored within a transmission case, whereby the meshing surfaces are lubricated.
In the method in which lubricating oil is stored within the transmission case, however, a resistance generated when the change-speed gears stir the lubrication oil cannot be ignored, and there is also caused a problem that the deterioration of lubricating oil is promoted.
Then, as is described in, for example, JP-B-6-27494, there is provided an engine lubrication system in which lubricating oil is supplied directly to meshing surfaces of the change-speed gears. In this related art, a main shaft 13 and a drive shaft 14 are disposed on separate mating surfaces of a crankcase 1 which is divided vertically into an upper case 2 and a lower case 3, a by-pass tube 36 for establishing communication between lubricating oil passageways 34 and 35 which are formed in axial side walls is arranged in parallel with the main shaft and the like in the upper case 2, and an injection port 38 is formed in the by-pass tube 36 for supplying lubricating oil to groups of change-speed gears.
In this related art described in the Japanese Examined Patent Publication, however, since a construction is adopted in which the by-pass tube 36 constituted by a separate part is disposed, the number of parts is increased by the addition of the by-pass tube 36, and a space where the by-pass tube 36 is disposed needs to be secured, leading to a possibility that the enlargement of the crankcase is called for. Furthermore, since a construction is adopted in which the by-pass tube 36 is inserted from the outside of the crankcase into the upper case 2, sealing needs to be provided between the by-pass tube 36 and an insertion hole, resulting in a problem that the construction is made complex.
The invention was made in view of the problems inherent in the related art and an object thereof is to provide an engine lubrication system which can implement the lubrication of meshing portions of change-speed gears of a transmission easily and securely without causing the problems that the construction is made complex and that the number of parts is increased.